This invention relates to a method and apparatus for injection molding, and more particularly a programmable method and apparatus for injection molding adapted to mold relatively thick products such as optical lenses for various applications.
According to a prior art method and apparatus for injection molding suitable for molding relatively thick products, the screw is rotated at its forward limit position so as to inject and fill a molten resin into the cavity of a metal mold, the shrinkage of the product is prevented and the dimensional accuracy thereof is ensured with an injection over a short stroke carried out for the purpose of pressure holding. According to another prior art method, while the screw is maintained at the most retracted position, the screw is rotated for supplementing a deficient quantity of the injected molten resin and then the screw is advanced for filling the mold cavity with the molten resin.
However, according to the first method the filling speed of the molten resin into the metal mold effected by the screw rotation is lower than that effected by the advancement of the screw so that the fluidity of the resin injected into the metal mold decreases because the injected resin comes into contact with the low temperature metal mold, thus forming weld marks or flow marks. Such tendency of forming weld marks and flow marks can be decreased by increasing the fluidity of the molten resin by increasing the mold temperature or the number of revolutions of the screw. These measures, however, elongate the cooling time as well as the molding cycle time. When the number of revolutions of the screw is increased, the resin, particularly such resin as polyvinyl chloride tending to undergo pyrolysis would be overheated to degrade the quality of the products. Furthermore, it takes a long time to fill the resin into the mold cavity so that at the time of transferring to the pressure holding step, the molten resin would solidify near the gate of the metal mold with the result that the holding pressure would not be applied to all portions of the mold cavity causing shrinkage of the molded products.
With the second prior art method, at the time of transferring to the injection operation (screw advancement) from the screw rotation, the flow of the molten resin injected into the metal mold stops momentarily so that a flow mark will be formed at that instant which impairs the optical characteristics of a lens, for example. Moreover, as the screw is rotated at its retracted position the effective screw length would be shortened substantially, thus decreasing the quantity of the resin injected. Moreover, as the molten resin is injected at a relatively low speed into the metal mold by the rotation of the screw, the thickness of the skin layer of the molded product becomes large, and since the high temperature molten resin is rapidly injected into the skin layer by the advancement of the screw, the bonding force between the injected molten resin and the skin layer is small, thus decreasing the mechanical strength of the molded product.
Furthermore, while products having a relatively large thickness are being molded by using such thermoplastic resins as hard polyvinyl chloride, acrylonitrile-butadiene-styrene copolymers which are difficult to burn, and polyoxide methylene with an injection molding machine of the type described above, when shortage or clogging of the resin occurs in a hopper, the resin would be decomposed by the heat generated by the rotation of the screw, thus resulting in products of poor quality. According to a prior art method a timer is provided which starts its time measuring operation and produces an alarm signal when an interval of screw rotation exceeds a predetermined time. However, when shortage of the resin occurs during the filling step the resin remaining in the heating cylinder undergoes heat decomposition due to the heat generated by the high speed rotation of the screw thus not only causing erosion of the inside of the heating cylinder and metal mold but also generating poisonous gas.